The present invention relates to a rolling mill and rolling method, and more particularly to a rolling mill and a rolling method for metal strip, in which relatively small-diameter work rolls suitable for rolling hard, thin material are used. The invention is applied to the type of mill in which a work roll is supported vertically and driven by a back-up roll, for example, a six-high mill having intermediate back-up rolls and outer back-up rolls or a four-high mill having no intermediate back-up rolls.
A rolling mill for rolling metal strip, particularly hard, very thin material such as stainless steel, high carbon steel, spring steel and some alloy steels such as titanium alloy and high nickel alloy steels, uses small-diameter work rolls. Since such work rolls have too small a diameter to allow direct application of the rolling torque to them, there have been developed multiple-roll rolling mills such as the Sendzimir mill and other mills in which the drive is transmitted to the work rolls via one or more pairs of back-up rolls. Methods have also been developed for controlling the bending of the work rolls in such rolling mills, in order to achieve flatness of the product, by relative shifting of the back-up rolls in the axial direction and also by applying vertical roll bending forces to the work rolls and the back-up rolls (see e.g. U.S. Pat. No. 4,369,646).
The present invention is concerned with control of bending in the horizontal rolling direction i.e. in the direction of travel of the material being rolled. This direction is referred to herein as the "horizontal direction" or "horizontal rolling direction" and these expressions do not include the axial direction of the rolls.
Since bending of the work rolls in the horizontal direction increases with decrease of roll diameter, there is a limit on the reduction of roll diameter. The roll bending phenomenon in the horizontal direction is discussed more below.
U.S. Pat. No. 4,631,948 discloses a rolling mill in which drive is transmitted to the work rolls by back-up rolls, and the work rolls are offset from the vertical axial plane of the back-up rolls in the horizontal direction. It is known that horizontal bending of the work rolls is reduced by offsetting the work roll axial plane from the back-up roll axial plane in the direction downstream (in the rolling direction) from the back-up roll plane because the frictional force applied by the back-up rolls to the work rolls is then in opposition to the horizontal component of the rolling force (i.e. the force applied to the material being rolled by the work rolls). In U.S. Pat. No. 4,631,948, the work rolls are maintained in a fixed horizontal position in the mill frame, offset relative to the back-up rolls, and are supported in the horizontal direction by support rollers which contact the work rolls at portions thereof which are outside the region contacting the rolled material but are of the same diameter as that region (i.e. the barrel diameter). The support rollers, which are on both sides of the work rolls in the horizontal direction, are forced against the work rolls hydraulically and serve to control horizontal bending, by applying bending forces to the rolls horizontally between their fixed bearing blocks. It is stated that the hydraulic cylinders which push the support rollers are independently controlled to produce the desired bending. However, in this mill because the bearing blocks are in a fixed horizontal position in the mill frame, appropriate control of the horizontal forces, which vary in dependence not only on the rolling direction but also various other factors during rolling such as torque and rolling force, is not possible.
JP-A-63-60006 (1988) shows an arrangement closely similar to that of U.S. Pat. No. 4,361,948, in which again the bearing blocks of the work rolls are horizontally fixed during rolling.
JP-A-60-18206 (1985) shows a similar application of rollers to both sides of both ends of both work rolls, to provide horizontal support of the work rolls. In this case the rollers which are paired are applied by a mechanical adjustment system against the work rolls. All of the rollers are apparently adjustable in the horizontal direction, but there is no suggestion of control of the horizontal position of the work rolls which are shown with their axes in the vertical plane of the axes of back-up rolls. It is stated that the Journal bearings of the work rolls may be removed, presumably since all horizontal force is controlled by the rollers. The mechanical adjustment system shown is not suitable for application of roll-bending forces during rolling. This prior art disclosure suggests no solutions to the problems of control of horizontal roll bending.
Control of bending of the work rolls across the whole width of the work rolls is provided by a system of support rollers or bearing rollers, such as in a Sendzimir rolling mill mentioned above. While such an arrangement provides good horizontal support of the work roll, it has the problem that the presence of the spaced bearings causes marks on the work roll, leading to transfer marking of the rolled product. Another problem is that the support rolls interfere with cooling of the work rolls.
U.S. Pat. No. 4,691,548 describes a rolling mill, for example a four-high mill, in which inner and outer bearing blocks on reduced diameter journal portions of the work roll are independently adjustable in the horizontal direction by hydraulic piston-and-cylinder adjustment units. The aim is stated to be to compensate for horizontal forces and/or for strip thickness regulation while maintaining the horizontal bending curve of the work rolls required for planeness of the strip. Continuous calculation of the required settings of the adjustment units and corresponding adjustment is mentioned. A problem with such an arrangement is the high bending moment which must be applied to the reduced-diameter portion of the roll, and it is stated in this prior disclosure that this bending moment can be reduced by applying bending forces acting on the outer bearings in the direction of the linear load exerted by the horizontal rolling force, but at the same time this increases the stress on the inner bearings. Conversely, when the bending forces exerted by the outer bearings act in the opposite direction, the bearing stress of the roll is reduced, but the bending moments at critical locations of the rolls are increased. The document apparently fails to resolve this problem, and furthermore does not apparently seek to minimize roll bending at the rolling region.
EP-A-416880 (corresponding to Japanese patent applications Nos. 231602/89 and 235518/90) aims specifically to minimize bending of the work roll at the rolling region, and describes a mill in which there are support rollers contacting the work roll outside the rolling region at barrel diameter on both horizontal sides of the work roll, acting both to locate the work roll at the desired offset horizontal position (relative to the back-up roll plane) and to support the work roll against the horizontal rolling forces. Particularly when the support rollers have a greater axial length, it is considered that in this manner the effective rigidity of the work roll is improved, so that horizontal bending is reduced.
Further work by the present inventors has shown that in the support roller system of EP-A-461880 described above, the effective rigidity of the work roll can be improved up to only about half as much the rigidity obtaining in the state of a wholly rigid horizontal holding of the work roll portions outside the rolling region (called "rigid support" below), due to elastic deformation of the surface of the support rollers and their axial (hub) portions in whichever way the support is effected by a plurality of support rollers. Even if the horizontal deflection of the work rolls can be limited to a low level by the conjoint use of the reduction of the horizontal force by the offset of the work rolls, such an arrangement alone limits the possible reduction of the diameter of the work rolls. Moreover, this technique describes only the method of reducing the horizontal force and reducing the deflection of the work rolls when the horizontal force is applied, but does not consider an instability phenomenon arising with rolls of very much reduced diameter resulting from the interaction between the rolling load applied to the work rolls and the horizontal deflection. It does not at all describe means for preventing this instability phenomenon and making it possible to carry out stable rolling.
In order to carry out stable rolling, it is necessary to consider how important is the role which the horizontal deflection rigidity of the work rolls plays and what impedes the maximum improvement of this effective rigidity. However, the prior art as a whole has not sufficiently taken these factors into consideration and has therefore failed to accomplish maximum possible reduction of the diameter of the work rolls.
It is an object of the present invention to provide a rolling mill and rolling method which can further increase the effective rigidity of the work rolls and thereby can permit reduction of the diameter of the work rolls.
According to the invention in a first aspect, there is provided a rolling mill having a work roll, a back-up roll for supporting the work roll vertically and driving the work roll, and a plurality of horizontal support rollers contacting the work roll at barrel diameter outside the rolling region and at both horizontal sides of the work roll and acting to fix the position of the work roll in both horizontal directions during rolling and to oppose horizontal rolling forces. The rolling mill is characterized by means for applying horizontal counterbending forces to the work roll comprising members contacting the work roll at locations axially further from the rolling region than said support rollers and actuator means for urging the members against said work roll. The counterbending forces act in the same direction as the net horizontal force applied to the work roll by the back-up roll and the material being rolled.
The effect of counterbending forces is, in combination with the support rollers, to reduce the horizontal bending of the work roll, thereby increasing the effective rigidity of the work roll against rolling forces in the horizontal direction.
It is desirable to provide control of the counterbending forces during rolling. Preferably, therefore, the mill has sensing means for sensing at least one condition of the work roll during rolling, and control means acting during rolling to control the means for applying counterbending forces in dependence on the sensed condition. The condition or conditions sensed by the sensing means is selected from (i) horizontal deflection of the work roll and (ii) the net horizontal force applied to the roll by the back-up roll and the material being rolled.
Preferably the horizontal rolling forces applied to the work roll during rolling are balanced substantially only by forces applied by the support rollers and the means for applying counterbending forces.
Preferably, the members contacting the work roll of said means for applying counterbending forces comprise a plurality of counterbending rollers contacting the work roll at barrel diameter, and the actuator means move these counterbending rollers in the horizontal direction relative to the support rollers. To mount the support and counterbending rollers, preferably the mill has, at each horizontal side of the work roll, a rigid support member carrying the support roller or rollers and counterbending roller or rollers, the rigid support members being movable in order to adjust the horizontal position of the work roll, i.e. to provide a desired offset relative to the back-up roll.
In order to achieve accurate location of the work roll at a desired horizontal position, preferably at one horizontal side of the work roll, the support rollers are carried by first support means providing during rolling a predetermined horizontal position of the support rollers carried thereby and at the other horizontal side of the work roll the support rollers are carried by second support means. The rolling mill further has force-applying means acting on the second support means so as to apply a predetermined horizontal force to the work roll, via the support rollers, urging the work roll against the first support means.
In another aspect, the invention provides a rolling mill having a pair of opposed work rolls, a pair of back-up rolls for respectively supporting the work rolls vertically and driving the work rolls, a plurality of horizontal support rollers contacting the work rolls at barrel diameter outside the rolling region and at both horizontal sides of the work rolls and acting to fix the position of the work rolls in both horizontal directions during rolling and to oppose horizontal rolling forces. There are respective means for applying horizontal counterbending forces to the two work rolls comprising, in each case, members contacting the work roll at locations axially further from the rolling region than the support rollers and actuator means for urging said members against the work roll. The counterbending forces act in the same horizontal direction as the net horizontal force applied to the work roll by the respective back-up roll and the material being rolled. There are further provided control means arranged for controlling the respective actuator means to apply the counterbending forces to the respective the work roll independently of the counterbending forces applied to the other work roll, so that for each work roll the counterbending forces applied are in the same horizontal direction as the net horizontal force.
In yet another aspect, the invention provides a method of control of a rolling mill in which a work roll is supported vertically and driven by a back-up roll and is positioned horizontally and supported horizontally by support rollers contacting the work roll at locations at barrel diameter outside the rolling region. The method is characterized by, during rolling, applying counterbending forces at locations axially outside or inside the support rollers in dependence on at least one of the quantities (a) horizontal deflection of the work roll and (b) horizontal force acting on the work roll, the counterbending forces acting in the same direction as the net horizontal force applied to the work roll by the back-up roll and the rolled material. The method preferably further includes shifting said work roll horizontally to a predetermined position for rolling by moving said support rollers.
The invention provides a method of operation of a rolling mill in which a work roll is supported vertically and driven by a back-up roll, the method comprising locating the work roll horizontally and supporting it against horizontal rolling forces by means of support rollers contacting the work roll at a barrel diameter outside the rolling region and applying horizontal counterbending forces tending to reduce horizontal bending of the work roll by means of counterbending rollers also contacting the work roll at a barrel diameter at locations axially further or closer from the rolling region than the support rollers, the counterbending rollers being movable in the horizontal direction relative to the support rollers.
In another aspect, the invention provides a method of control of a rolling mill in which two opposed work rolls are supported vertically and driven by respective back-up rolls, the method comprising during rolling controlling horizontal bending of the two work rolls so as to reduce bending of each roll by applying horizontal roll-bending forces to the two work rolls independently in dependence on at least one sensed condition of each work roll.